As it is well known, a LED (acronym from: “Light-Emitting Diode”) is an electronic light source based on a semiconductor diode. When the diode is forward biased or switched on, electrons are able to recombine with holes and energy is released in the form of light, according to the electroluminescence effect, the colour of the light being determined by the energy gap of the semiconductor material composing the diode, i.e., the LED.
A LED lamp usually comprises clusters of LEDs in a suitable housing, arranged according to different shapes, such as a standard light bulb shape.
In particular, in the lighting field, some strongly feel the need of replacing standard lamps, such as classical incandescent bulb lamps, with semiconductor light sources, in particular LED lamps, LEDs presenting many advantages over traditional light sources including lower energy consumption, longer lifetime, improved robustness, smaller size, and faster switching time. Such a replacement is namely recommended, e.g., in lighting apparatuses for bars, hotels, and other commercial sites requiring a constant lighting over long periods of time.
However, LED lamps may require more precise current and heat management than traditional light sources, and a main challenge is the huge ratio between the forward voltage required by the LEDs of such lamps and the AC mains voltage supplying the lighting apparatus.
AC LED lamps are thus usually driven by an appropriate driving circuit, which substantially comprises a converter and usually employs at least a transformer.
Different solutions for a driving circuit of this type are commonly used, aiming to correctly supply LEDs from an AC mains voltage reference.
A first very simple solution is that of using a so called step-down or buck converter, as schematically shown in FIG. 1, globally indicated at 1.
The buck converter 1 has a first I1 and a second input terminal I2 connected to the AC mains, and a first O1 and a second output terminal O2 connected to a semiconductor light source, in particular a LED lamp LL.
The buck converter 1 comprises a PWM controller 2 connected to the first input terminal I1 by means of a resistive path, namely a series of a first R1 and a second resistor R2, as well to a voltage reference, namely a ground GND, by means of a first capacitor C1. The PWM controller 2 also has an output node Xout connected to a control or gate terminal of a transistor M1, in particular a Power MOS transistor, in turn inserted between a first internal circuit node X1 and ground GND.
Moreover, the first internal circuit node X1 is connected to the first output terminal O1 of the buck converter 1 by means of a Zener diode DZ as well as to the second output terminal O2 of the buck converter 1 by means of a filtering inductor L1.
The buck converter 1 also comprises a rectifier bridge 3 being connected to the first and second input terminals, I1 and I2, as well as to a second and a third internal circuit node, X2 and X3. The second internal circuit node X2 is also connected to the first output terminal O1 of the buck converter 1 and to the third internal circuit node X3 by means of a second capacitor C2, the third internal circuit node X3 being in turn connected to ground GND.
The operation of a buck converter is fairly simple, with two switching elements (namely the transistor M1 and the Zener diode DZ) that alternatively connect the filtering inductor L1 to the power supply reference to store energy and discharge it into the load, i.e., the LED lamp LL.
By using a buck converter 1 of this type, a driving circuit is obtained which shows a very good efficiency of conversion and which is thus mainly suitable for driving LEDs. However, when the difference between the rectified AC mains voltage and a forward or supply voltage (VFLEDs) to be provided to the LEDs of the LED lamp LL is relatively big, the efficiency of this known buck converter 1 falls abruptly.
Also, the filtering inductor L1 is not small and the area required by the buck converter 1 as a whole is thus medium, the same working with a switching frequency in the range of 100 kHz.
In view of all its features, a buck converter 1 of this type can thus provide a driving circuit for a semiconductor light source, in particular a LED lamp, having a medium cost.
An other solution is that of using a so called fly-back converter, as schematically shown in FIG. 2, globally indicated with 5.
In particular, the fly-back converter 5 has a first IN1 and a second input terminal IN2 connected to the AC mains, and an output terminal OUT1 connected to a semiconductor light source, in particular a LED lamp LL, in turn connected to a feedback node X7, whereat a feedback voltage VFB is applied. The current flowing through the LED lamp LL is sensed as a voltage drop on a resistor R8, which is connected between the feedback node X7 and a ground GND.
The fly-back converter 5 comprises a fly-back controller 6 having a first input node X5 being connected to the input terminal IN by means of a resistive path, namely by a series of a first and a second resistor, R5 and R6, as well as to ground GND, by means of a first capacitor C5, and a second input node X6 connected to an optocoupler 7, in turn connected to ground GND.
In particular, the optocoupler 7 comprises an emitting diode LED and a phototransistor PT, separated so that light may travel through a barrier but an electrical current may not. When an electrical signal is applied to the input of the optocoupler 7, i.e., to the second input node X6, the phototransistor PT switches on and the emitting diode LED lights and a corresponding electrical signal is generated at the output of the optocoupler 7, i.e., to the feedback node X7. Such a feedback node X7 is also connected to the resistor R8, which is used to sense the current flowing through the LED lamp LL.
The fly-back converter 5 further comprises a transformer 8, having a first, a second and a third winding, TR1, TR2 and TR3. In particular, the first winding TR1 is inserted between a second internal circuit node X8 and a second input node X9 of the fly-back controller 6 and it is coupled to the second and third windings, TR2 and TR3 of the transformer 8. The second internal circuit node X6 is also connected, by means of a second capacitor C6, to ground GND.
Moreover, the second winding TR2 is connected to the output terminal OUT1 of the fly-back converter 5 by means of a first diode D1 and to ground GND, a second capacitor C7 being also connected between the output terminal OUT1 and ground GND.
The third winding TR3 is connected to the first input node X5 of the fly-back controller 6 by means of a second diode D2 and to ground GND, the first capacitor C5 being connected in parallel to such a third winding TR3 between the first input node X5 and ground GND.
Finally, the fly-back converter 5 comprises a rectifier bridge 9 connected to the first and second input terminals, IN1 and IN2, of the fly-back converter 5, to the second internal circuit node X8 and to ground GND.
It should be noted that the fly-back converter 5 is able to reduce the large ratio between the AC mains voltage and the forward or supply voltage (VFLEDs) to be provided to the LEDs of the LED lamp LL, essentially thanks to the transformer 8.
However, this solution is relatively complex, mainly due to the presence of the transformer 8.
Advantages of this known fly-back converter are a very good efficiency of conversion, a wide range of the forward voltage (VFLEDs) for the LEDs, and a wide range of output currents, working with a switching frequency in the range of 100 kHz.
Moreover, the fly-back converter 5 is able to insulate the LED lamp LL from the AC mains in case of breakdown, thanks to the transformer 8 and to the optocoupler 7.
However, the fly-back converter 5 has a fairly high number of components, the biggest ones being the transformer 8, the rectifier bridge 9, the fly-back controller 6 and the optocoupler 7. So, this solution also has many drawbacks, namely high costs, a high area occupation and a high complexity.